When using downhole tools that require a great degree of accuracy as regards axial feeding, for example using cutting tools when working a casing, it often presents large problems to do this work accurately enough. Axial feeding takes place, to a great extent, by a pipe string being moved forwards or being withdrawn while the tool is working, and this may easily result in the tool being overloaded so that the entire pipe string will have to be pulled up for maintenance or replacement of the tool. This involves large costs by the very fact that a pipe string of this kind may have a considerable length, especially in subsea oil and gas production and when wells with horizontal portions are used.